Ionization gauges, more specifically Bayard-Alpert (BA) ionization gauges, are the most common non-magnetic means of measuring very low pressures. The gauges have been widely used worldwide. These gauges were disclosed in 1952 in U.S. Pat. No. 2,605,431, which is herein incorporated by reference in its entirety. A typical ionization gauge includes an electron source, an anode, and an ion collector electrode. For the BA ionization gauge, the electron source is located outside of an ionization space or anode volume which is defined by a cylindrical anode screen. The ion collector electrode is disposed within the anode volume. Electrons travel from the electron source to and through the anode, cycle back and forth through the anode, and are consequently retained within, or nearby to, the anode.
In their travel, the electrons collide with molecules and atoms of gas that constitute the atmosphere whose pressure is desired to be measured. This contact between the electrons and the gas creates ions. The ions are attracted to the ion collector electrode, which is typically connected to ground. The pressure of the gas within the atmosphere can be calculated from ion and electron currents by the formula P=(1/S) (Iion/Ielectron), where S is a coefficient with the units of 1/Torr and is characteristic of a particular gauge geometry, electrical parameters, and pressure range.